In an image-forming apparatus wherein an image (latent image) is formed on an image-bearing member such as a drum or belt which is movable along an endless path, it is required that the speed of the movement of the surface of the image-bearing member (also referred to as the "load") is maintained at a predetermined speed. If the surface speed is different from the predetermined speed, or if the surface speed is not uniform, the image formed on the image-bearing member is contracted or expanded, whereby the formed image is distorted.
U.S. Pat. No. 4,761,662 attempts to control the load velocity by sensing and controlling the velocity of the load directly. Use of this mechanism maintains a constant rotational speed of the drum output. This method assumes that mechanical constraints allow placement of a suitable velocity and/or position sensor at the load. Also assumed is that transmission nonlinearities do not result in control instabilities and that the cost of a separate sensor with reasonable resolution is not prohibitive.
The prior art is replete with techniques for controlling the velocity of a belt, where a load is driven through a belt transmission by a servo controlled motor. The velocity of the load is controlled by controlling the velocity of the motor. For example, if a constant velocity is desired from the load, the motor is servo driven at a constant velocity. Because any transmission system has compliance, it will oscillate or "ring" if torques are applied at the load; the motor will also receive a measure of the torque disturbance, but the servo system will substantially eliminate any velocity variation at the motor. It should be noted that the motivation was to provide constant velocity at the load; however, in practice, the compliance of the transmission results in velocity variations at the load.
Unstable surface speeds can result from a deformation of the transmission which transmits the driving force from the motor to the drum. Because the surface speed of the drum is not controlled, but is allowed to change, the sharpness of the image formed on such a surface is degraded.
A potential solution would be to drive the motor based on the velocity of the load. A couple of problems arise; first sensing the load velocity is often much more difficult than sensing the motor velocity due to mechanical constraints. Secondly, nonlinearities in the transmission can cause control instabilities.
U.S. Pat. No. 5,076,568 attempts to eliminate oscillations in an endless belt by introducing a second motor to drive the load motor in such a way that velocity variations of the belt are eliminated.
U.S. Pat. Nos. 4,541,709 and 4,723,145 both refer to methods of minimizing the velocity errors between an image-bearing member and a transfer material-bearing member. Both use a motor for each of the movable members. Neither addresses the effect of compliance in the transmission between either motor and its load. The present invention serves to minimize the effects of this compliance through the use of a single motor and measurement of the relative states between the motor and the image-bearing member.